Geophysical prospecting methods can be classified as: -Natural methods, as gravity and magnetic methods -Artificial methods, as seismic and some electric methods

  • It can also be classified as:

-Deep geophysical methods as gravity, magnetic, and seismic reflection methods.

-Shallow geophysical methods as electric, seismic refraction and GPR methods.

Electrical Prospecting

Electrical prospecting methods include two types:

- Natural methods as SP and telluric currents methods –

- Artificial methods as the resistivity, IP and EM methods.

Importance of Electrical Methods

Most of electric methods are effective only in shallow exploration and they can be used in the following aspects:

Searching for metals and minerals.

The prospecting for under ground water.

Engineering geology to determine the depth of bedrock.

Geothermal exploration.

Electrical properties of rocks

Electrical prospecting makes use of three important properties of rocks:

  • The resistivity which governs the amount of current that path through a rock when specified potential difference is applied.
  • The electrochemical activity with respect to electrolytes in the ground is the base for the self potential and induced polarization methods.
  • The dielectric constant gives information on the capacity of a rock material to store electric charge as in inductive prospecting methods.

Resistivity

  • The electric resistivity (p) of any material is defined as the resistance (R) of a cylinder with cross section of unit area (S) and with unit length (L).

r = R S / L     ……………….……..(1)

  • The unites of resistivity in the metric system is the Ohm. cm. The current (I) is related to the impressed voltage (V) and the resistance (R) by Ohm’s low

R = V / I ………………………….(2)

From equations 1 and 2,

r = (S/L).V/I …………………..(3)

Resistivity ranges

  • The resistivity ranges for rocks and minerals (Fig. l) extend from 10-5 to 1015 Ω.m. as follow:

- Good conductors    10-5      to 10-1.

- Intermediate conductors    1 to 107.

- Poor conductors     108 to 1015.

  • In porous sedimentary rocks the resistivity is governed more by the electrolyte concentration of the liquid filling the pores interstices within the formation.

 

Fig. (l): Approximate ranges for the electric resistivity of rocks and soils.

 

Electrochemical activity

  • The electrochemical activity of rocks depends on:

-The chemical composition of rocks

-The chemical composition and concentration of the electrolytes with which these rocks in contact

  • This activity governs the magnitude and sign of the voltage developed when the rock material is in equilibrium with the electrolyte.

Dielectric constant

  • The dielectric constant is a measure of the polarizability of a material in an electric field.
  • The polarization, or electrical moment (P) per unit volume is proportional to the impressed electrical field (E) and the proportionality constant is (X), the electrical susceptibility.

P = XE

  • The total electrical flux per unit area (corresponds to magnetic flux density) is:

E + 4pP = E + 4pXЕ = E (1 + 4pX)

  • The quantity (1 + 4pX) is designated as “€”, the dielectric constant, which is analogous to magnetic permeability.
  • The dielectric constant determines the effective capacitance of a rock material and its response to any applied electric field, either direct or alternating.
  • The dielectric constant is quite sensitive to temperature, the value of “€” increases as the rock become hotter.

 The dielectric constant ranges are as follows:

  • For most hard rocks about 6 to 16 eus
  • For wet soils it is greater up to 40 or 50 eus
  • For the vacuum it is unity

Note : eus (electrostatic unit)

 

كتب مقترحة

1- Milton B. Dobrin  Carl H. Savit , 1988: Introduction to Geophysical Prospecting. Mcgraw-Hill College; 4 Sub edition (March 1988)

2- Kearey, P. and Brooks, M., An Introduction to Geophysical Exploration, 2nd Edition: Blackwell Science Pub., 1991.

3- Exploration Geophysics, Published 1997 by Baffins Lane, Chichester, West Sussex PO19 1UD, England

  • Applied and environmental geophysics, 1999, Sharma,V.,P.
  • Introduction to geophysical prospecting, 1988, Durbin, M. B.
  • Geophysics.net
  • hager-richter.com/resistivity.htm
  • geovision.com/PDF/M_Resistivity.pdf

 

اسم المقرر:جيوفيزياء II

 

أهداف المقرر:

 تزويد الطالب بالطرق المختلفة لقياس وتصحيح وتفسير البيانات الجيوفيزيائية المختلفة (جاذبية ومغناطيسية وسيزمية وكهربية) لمعرفة أنواع الصخور والمعادن والتراكيب الجيولوجية بمناطق الدراسة -  يفسر البيانات الجيوفيزيائية وذلك باستخدام برامج الكمبيوتر.

محتوى المقرر:

1- الطرق التناقلية: قياس وتصحيح البيانات التثاقلية- تغير الجاذبية مع الزمن – نظريات توازن القشرة الأرضية وسمكها – مقدمة عن تفسير البيانات التناقلية

2- الطرق المغناطيسية: قياس وتصحيح البيانات المغناطيسية – المغناطيسية الأرضية وتغيرها مع الزمن - مقدمة عن تفسير البيانات المغناطيسية

3- الطرق السيزمية: نبذة مختصرة عن الزلازل – أنواع الموجات السيزمية – طريقتي الانعكاس والانكسار وتفسيراتها.

4- الطرق الكهربية: الخصائص الكهربية للمعادن والصخور – الطرق الكهربية المختلفة مقدمة عن تفسير البيانات الكهربية